Oscillating water columns (OWCs) are well-known devices for extracting energy from waves, and comprise a chamber having a bottom which is open and underwater and with an opening at the top, above water, the chamber housing or communicating with one or two air turbines with or without non-return valves, arranged in such a way that a turbine is turned so as to produce power regardless of whether air is flowing into or out of the chamber consequent upon wave motion. A preferred method is to use one bi-directional turbine (i.e. one that turns in the same direction regardless of the direction of air flow). As a wave crest approaches, the water surges up inside the chamber, reducing the air volume above the water surface and increasing the pressure. Air therefore flows outwards through the turbine, generating power. As the wave subsides the air space increases, the pressure drops and air is drawn back into the chamber through the turbine, again generating power. This process is repeated with each wave.
OWC devices have been constructed on exposed rocky coastlines, however these onshore devices may be opposed by the public on the grounds of visual pollution. Also, wave energy is attenuated in shallow water and an OWC in deep water (>50 m) is able to harvest more energy.
Floating prototype OWC devices have been tested with the aim of harvesting energy from the more energetic wave climate in deeper water. One of these devices, the Kaimei, was based on a small ship hull. The efficiency of conversion of wave energy in this device however was found to be low because the floating body was too small and light for the prevailing wave period and tended to move up and down with the swell, cancelling out most of the relative movement between the body and the surrounding water. Another device, the Osprey, was not based on a ship hull and was not able to weather a storm when it was being towed into position. A third device, the Swedish Floating Wave Power Vessel, is a large, expensive purpose-built structure which only captures energy from a small length of wave front, so that the capital cost per unit of rated output is high.
A further problem with known OWC devices is that the fixed vertical wall of the chamber facing the oncoming swells shuts out much of the available energy because the water moves in an orbital motion, with the water at the wave crest advancing in the direction of motion of the wave. As a result, the highest known water to air energy conversion efficiency for an OWC is just under 70%.
Present day wind turbines are a mature technology, and the 3 blade, horizontal axis type has become the industry standard. These wind turbines have the gearbox and generator assembly housed in a nacelle at the top of the tower, and in large, multi-megawatt machines this assembly typically weighs hundreds of tonnes. Blades may be 40 m or more in length. As the size of wind turbines increases, it is becoming increasingly difficult to transport such large components to remote hilly onshore sites and lift them into position on top of towers, which may be 60 to 100 m high. This transport difficulty, and the fact that the best onshore sites on some countries are either already taken, or are not available for environmental or social reasons, has driven a move to develop offshore wind farms. While wind energy offshore is typically 30-50% higher than at onshore sites, foundations, installation and maintenance are more expensive. Wind turbines fixed to the sea floor are limited to depths of about 15 m, and can only be installed in relatively calm weather, which is not common at sites selected for their consistent high winds. The massive gearboxes needed for these machines are reaching the limits of manufacturing and installation capacity, and a large proportion of wind turbine failures are related to gearboxes. There is now speculation in the wind energy industry that there may be a need for a radically different form of wind turbine, especially for larger offshore installations, which does not suffer from the problem of very large, heavy gearboxes which must be mounted and serviced high off the ground.
Marine current turbines of both axial flow and cross flow patterns are being developed by a large number of companies around the world. These require expensive mounting structures to withstand the large hydrodynamic forces which act on them, and the cost of fabrication and installation of these support structures is a major problem, reducing cost-effectiveness.